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Fabrication ProcessCrystal Growth

DopingDeposition

Patterning

Lithography

Oxidation

Ion Implementation

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Fabrication- CMOS Process

Starting Material Preparation

1. Produce Metallurgical Grade Silicon (MGS)

SiO2 (sand) + C in Arc FurnaceSi- liquid 98% pure

2. Produce Electronic Grade Silicon (EGS)

HCl + Si (MGS)Successive purification by distillationChemical Vapor Deposition (CVD)

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Fabrication: Crystal Growth

Czochralski Method

Basic idea: dip seed crystal intoliquid pool

Slowly pull out at a rate of0.5mm/mincontrolled amount of

impurities added to melt

Speed of rotation and pullingrate determine diameter of theingotIngot- 1to 2 meter longDiameter: 4, 6, 8

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Fabrication: Wafering

Finish ingot to precise diameter

Mill flats Cut wafers by diamond saw:

Typical thickness 0.5mm

Polish to give optically flatsurface

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Fabrication: Oxidation

Silicon Dioxide has several uses:- mask against implantor diffusion

- device isolation- gate oxide- isolation between layers

SiO2 could be thermally generated or through CVD

Oxidation consumes silicon

Wet or dry oxidation

Quartz Tube

Wafers

Quartz Carrier

Resistance Heater

O2or Water

VaporPump

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Fabrication: Diffusion Simultaneous creation of p-n junction over

the entire surface of wafer Doesnt offer precise control

Good for heavy doping, deep junctions Two steps:Pre-depositionDopant mixed with inert gas introducedin to a furnace at 1000 oC.Atoms diffuse in a thin layer of Si surfaceDrive-inWafers heated without dopant

Resistance Heater

wafers

Temp: 1000

Dopant Gas

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Fabrication: Ion

ImplantationPrecise control of dopantGood for shallow junctions and threshold adjustDopant gas ionized and acceleratedIons strike silicon surface at high speed

Depth of lodging is determined by accelerating field

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Fabrication: Deposition

Reactant0.1 -1 Torr

Loader

Pump

Used to form thin film of Polysilicon,Silicon dioxide, Silicon Nitride, Al.

Applications: Polysilicon, interlayeroxide, LOCOS, metal.

Common technique: Low PressureChemical Vapor Deposition (CVD).

SiO2 and Polysilicon deposition at300 to 1000 oC. Aluminum deposition at lower

temperature- different technique

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Fabrication:

Metallization Standard material is Aluminum

Low contact resistance to p-type and n-type

When deposited on SiO2, Al2O3is formed: good adhesive

All wafer covered with Al

Deposition techniques:

Vacuum EvaporationElectron Beam EvaporationRF Sputtering

Other materials used in conjunction with or replacement to Al

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Fabrication: EtchingWet Etching Etchants: hydrofluoric acid (HF), mixture of nitric acid and

HF Good selectivity Problem:

- under cut- acid waste disposal

Dry Etching Physical bombardment with atoms or ions good for small geometries. Various types exists such as:

Planar Plasma EtchingReactive Ion Etching

Plasma Reactive species

RF

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Fabrication:

LithographyMask making

Most critical part of lithography is conversion from layout tomaster mask

Masking plate has opaque geometrical shapes correspondingto the area on the wafer surface where certain photochemicalreactions have to be prevented or taken place.

Masks uses photographic emulsion or hard surface

Two types: dark field or clear field

Maskmaking: optical or e-beam

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Fabrication:

Lithography

Step & Repeat

Printing

Printing

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Lithography: Mask

makingElectron Beam Technique

Main problem with optical

technique: light diffraction

System resembles ascanning electron microscope +

beam blanking and computercontrolled deflection

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Patterning/ PrintingProcess of transferring mask features to surface of the silicon wafer.

Optical or Electron-beam

Photo-resist material (negative or positive):synthetic rubber orpolymer upon exposure to light becomes insoluble ( negative ) orvolatile (positive)

Developer: typically organic solvant-e.g. Xylen

A common step in many processes is the creation and selectiveremoval of Silicon Dioxide

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Patterning: Pwell mask

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Patterning/ Printing

substrate

SiO2

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Fabrication Steps

Apply PR

Pre-bake

Printer align exposemask

Develop, rinse, dry

Post bake

Inspect, measure

Etch

Strip resist

Deposit or grow layer

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Fabrication Steps

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Fabrication Steps: P-

well ProcessDiffusion

VDD

Vo

P well

P+

P+

n+

n+

Vin

p+

P well

p+p+ p+n+ n+

Substrate n-type

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Fabrication Steps: P-

well ProcessDiffusion

VDD

Vo

P well

P+

P+

n+

n+

Vin

p+

P well

p+p+ p+n+ n+

Substrate n-type

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Fabrication Steps

n+n+

P well

p+ p+

Substrate n-type

P well

n+ n+p+ p+

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Fabrication StepsOxidation

Substrate n-type

Patterning of P-well mask

Substrate n-type

oxide

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Fabrication StepsDiffusion: p dopant,Removal of Oxide

Deposit Silicon Nitride

P-well

P-well

Si3N4

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Fabrication StepsPatterning: Diffusion (active)mask

Oxidation

P-well

substrate

FOXFOX FOX

substrate